For umpteen centuries people thought the Earth was the center of the cosmos. The Sun and everything else revolved around it. In the 2nd century AD it was the foundation for Ptolemy's Almagest, and it persisted into the 18th century. But it wasn't unchallenged, there was a revolution in the making. Here are some of the heroes of this revolution. They have statues and plaques, but for the 21st century, each has had a Google doodle too.

Nicolaus Copernicus (1473-1543)
Copernicus was an administrator in a cathedral in northern Poland, but informally he was also an astronomer and mathematician, trained in Bologna, Italy. He found the geocentric (Earth-centered) cosmos cumbersome. It had planets moving in little circles called epicycles around bigger circles, and couldn't explain why planets occasionally orbited backwards (retrograde motion).

But if the Sun were at the center, retrograde motion made sense. It would mean Earth in its orbit was overtaking another planet in its orbit. In his great work De Revolutionibus he worked hard to provide arguments and calculations for a heliocentric (Sun-centered) system. But even his system didn't quite get the planets in the right places, so there were still epicycles.

On February 19, 2013, the 540th anniversary of his birth, a doodle represented the heliocentric Solar System.

Tycho Brahe (1546-1601)
Tycho was not a Copernican. He had his own hybrid system with the planets orbiting the Sun, and the Sun and Moon orbiting the Earth. His contribution to the revolution wasn't his theory, but his data. Tycho had his own observatory in Denmark, and he was the most methodical and accurate observer of the era before telescopes. His years of recorded observations constituted a unique database.

On December 14, 2013 – Tycho's 467th birthday – he was depicted pointing to the brilliant nova he had studied.

Johannes Kepler (1541-1650)
Kepler was an excellent mathematician, and Tycho hired him as an assistant. After Tycho's death he used the master's observations to try to make sense of the planetary orbits. Eventually, he could almost fit the data to a Copernican system. Yet not quite. But he had too much respect for Tycho's accuracy to think that almost was good enough. Then he found the flaw in the Copernican model: it was based on circular orbits. What if the orbits were ellipses? (These are squashed circles with two focal points.) Using Tycho's data and elliptical orbits with the Sun at one focus gave a heliocentric Solar System that worked. Kepler's math showed how a planet's velocity and its orbital period related to its distance from the Sun.

A tribute appeared on December 27, 2013 on the 442nd anniversary of Kepler's birth.

Galileo Galilei (1564-1642)
Galileo was an avowed Copernican, and he provided an essential piece of evidence supporting the theory. He observed the planet Venus with his telescope, and discovered that it had phases like the Moon does. That couldn't happen in the Ptolemaic system, but would occur in a heliocentric system if Venus's orbit were nearer to the Sun than Earth's.

Galileo didn't get a birthday doodle, but on August 25, 2009 a doodle celebrated 400 years of Galileo's telescope.

Isaac Newton (1643-1729)
Kepler described some of the dynamics of planetary orbits, but there were no physical principles known to underlie the arrangement of the Solar System. That ended up as a job for Newton's physics. Kepler's laws of planetary motion can be derived from Newton's theory of gravitation. And Newton's theory could be applied to any object, including comets, or the satellites of a planet. His Principia is one of the cornerstones of modern science, though not everyone eagerly embraced it at the time.

Newton was actually born on Christmas day 1642, but the equivalent date on the Gregorian calendar we now use is different. So January 4, 2010 was Isaac Newton's 367th birthday, noted by a doodle with a falling apple.

Edmond Halley (1656-1742)
Halley was a notable scientist, not only in astronomy but in several other fields. He was also a friend of Isaac Newton. He encouraged Newton to write the Principia, edited the final work, and paid for its publication. (He did recover the money from sales.) In addition, Halley's work on comets was based on Newton's theory. He was the first person to realize that a comet could return, and correctly predicted the return of the one that now bears his name. Although he didn't live to see it, it was a posthumous triumph for him and a validation of Newton's theories.

On November 8, 2011, Edmond Halley's 355th birthday, Halley's comet featured in a doodle.